The nuclear medicine diagnosis system, nowadays widely in use, comprises a radiation measurement device, which measures radiations such as gamma rays. Typical nuclear medicine diagnosis systems are a gamma camera, a Single Photon Emission Computed Tomography (SPECT), and a Positron Emission Tomography (PET). In addition, demand for radiation measurement devices such as the one used in dosimeters as countermeasures against radiation-bombing terrorism is rising in the field of the homeland security.
The radiation detectors mounted on the radiation measurement devices conventionally combines a Scintillator (a device which absorbs radiation energy and generates fluorescence) and a photo multiplier tube. Nowadays, technologies of applying semiconductor radiation detectors comprising semiconductor crystals such as cadmium telluride (CdTe), cadmium (Cd), zinc (Zn), tellurium (Te), gallium arsenide (GaAs), or thallium bromide (TlBr) have drawn attention as technologies of producing radiation detectors for detecting radiations such as gamma rays.
The semiconductor radiation detector converts electric charge to an electric signal, which charge is generated by a radiation interacting with a semiconductor crystal. The detector has various features such as better conversion efficiency than a scintillator-based detector, and a potential for downsizing of the detector.
In addition, the semiconductor radiation detector comprises: a semiconductor crystal; a cathode electrode on a surface of the semiconductor crystal; and an anode electrode opposed to the cathode electrode across the semiconductor crystal sandwiched between the cathode and anode electrodes. In the detector, a high DC voltage applied to the cathode electrode and the anode electrode generates electric charge caused by incoming radiations like an X-ray or gamma rays coming into the semiconductor crystal, then the electric charge is picked up as a signal from the cathode electrode or anode electrode.
Among the semiconductor crystals described above, a thallium bromide crystal in particular has a higher linear attenuation coefficient in the photoelectric effect than other semiconductor crystals such as cadmium telluride, cadmium, zinc, tellurium, and gallium arsenide. This enables a relatively thinner thallium bromide crystal to achieve the same level of gamma ray sensitivity as other semiconductor crystals. The thinner crystal can make the size of a radiation measurement device equipped with a semiconductor radiation detector configured with a thallium bromide crystal and the size of the nuclear medicine diagnosis system comprising such measurement device smaller than other radiation measurement devices equipped with other semiconductor radiation detectors and than other nuclear medicine diagnosis system comprising such other measurement devices.
In addition, thallium bromide is less expensive than other semiconductor crystals such as cadmium telluride, cadmium, zinc, tellurium, and gallium arsenide. The less expensive semiconductor crystal lowers the production cost of the radiation measurement device equipped with a thallium bromide base semiconductor radiation detector, and the production cost of the nuclear medicine diagnosis system comprising such measurement device, below the costs of other radiation measurement devices configured with other semiconductor radiation detectors, and below the cost of other nuclear medicine diagnosis systems comprising such other measurement devices.
Additionally, thallium layers are inserted between a thallium bromide crystal and both of the ordinary cathode electrode and anode electrode of a semiconductor radiation detector configured with a thallium bromide crystal. The insertion prevents a polarization and achieves a long stable operation due to the formation reaction of thallium metal and thallium bromide (see patent document 1 and non-patent document 1 for example).
Hereinafter, the polarisation means an aberration of a crystal structure or characteristics, and described in detail later.
In addition to the insertion of a thallium layer between the thallium bromide crystal and the cathode and anode electrodes of a semiconductor radiation detector, the polarity of the voltage for collecting electric charge applied to the detector may be reversed at regular time intervals. This achieves an even longer stable operation of the detector due to the reversible formation reaction of metal thallium and thallium bromide (see patent document 1 and non-patent document 2 for example).